Auxiliary Propulsion.-The ATS IV low thrust level resistojet system for spacecraft station keeping was designed so as not to disturb the sensitive passive attitude control system during station keeping. Although the spacecraft failed to achieve its design orbit, the 50-micropound-thrust ammonia resistojet system operated successfully for over 800 hours.

The ATS IV also carried a cesium ion engine experiment which functioned as intended. The system was capable of varying thrust level from 5 to 20 micropounds and of steering the thrust electrically. The steering capability eliminates unwanted torques by having the thrust vector pass through the spacecraft center of mass. Both the resistojet and ion engine systems will be flown on the ATS E.

Resistojets capable of operating in the 3600-4000°F range may also find application in position control of manned space stations. The 10 millipound units (19th Semiannual Report, p. 111) undergoing life tests have now accumulated about 5,000 hours on ammonia and 3,500 hours on hydrogen. In addition, research was started on such units capable of using life support waste products as propellants.

Prime Propulsion.-The SERT II project progressed as assembly of the flight spacecraft was completed. Planned for launch in 1969, SERT II is intended to demonstrate six month performance of a 1 kilowatt mercury bombardment ion engine in the space environment.

A ground test program to demonstrate the feasibility of solar powered electric propulsion systems also made substantial progress. All of the equipment (thrusters, gimbals, power conditioners, and switches) required was on hand and being assembled. The open-loop test program, set to commence in 1969, should provide data for an advanced closed-loop test involving automatic failure detection, switching, power distribution, and attitude control. The closed-loop program, scheduled for 1970, should in turn isolate and resolve the operational problems of such a system during a mission.

Temperature Control

SPACE VEHICLES PROGRAM

In laboratory tests of pigments for thermal control coatings, zinc titanate was found to be very stable in simulated space environment, but grinding the pigment damaged its structure and reduced its stability. To meet the need for powdered zinc

titanate suitable for use as a pigment, a method was developed for direct precipitation of the chemical from a gaseous mixture of zinc vapor and other chemical compounds in vapor form.

Thermal/Vacuum Test Technology

In research on methods of testing very large spacecraft in available thermal/vacuum test facilities, studies were being conducted to develop valid scale model test techniques. Procedures devised in testing a 1/6-scale model of a proposed space telescope under transient thermal conditions appeared to give more detailed and valid results than those achieved by analytical prediction techniques.

Lifting-Body Flight Program

The HL-10, which has now been flown 14 times by three pilots, used the XLR-11 rocket engine on the last three flights to boost the speed to approximately Mach 0.84. Vehicle performance has been good, and flight testing through the transonic speed range will continue.

The X-24A was undergoing ground checkouts in preparation for its first flight in 1969.

Advanced Guiding Parachutes

Nine flights of a 4,000-square-foot parawing were completed in the second flight test phase of the parawing technology program. The test flights, with payloads ranging from 2,900 pounds to 5,000 pounds, evaluated deployment characteristics under conditions simulating the deployment of large scale parawings with 15,000-pound payloads. The test flights are scheduled to continue, culminating in the testing of 10,000-square-foot wings and payloads up to 15,000 pounds.

Advanced Decelerator Concepts

A 40-foot-diameter supersonic parachute was successfully flight tested by the Langley Research Center at Mach 3.5 and an altitude of 33 miles. The objective of the test, in which the test vehicle and parachute were lofted to altitude by a three-stage rocket vehicle, was to measure drag and stability of the parachute under atmospheric density conditions simulating those believed to exist near the surface of Mars.

Apollo 6 Flight Anomaly

The Langley Research Center directly supported the Manned

Spacecraft Center investigation of the Apollo 6 (April 4, 1968) flight anomaly a noncatastrophic structural failure on the adapter section containing the Lunar Module (19th Semiannual Report, p. 18). The Langley investigation included static and dynamic analyses and model tests of the adapter section and its neighboring components as well as model structural tests of the complete vehicle. The Langley work, which provided a base from which MSC could plan and devise similar tests on the fullscale hardware for the most critical conditions, was particularly valuable in determining the appropriate location of instrumentation for full-scale tests.

Apollo Pressure Vessel Problems

During proof tests, a few of the many pressure vessels in the Apollo system developed deficiencies resulting in leaks and structural failures, which in flight could be catastrophic. Lewis Research Center research on these pressure vessel problems produced significant data on fracture behavior of pressure vessels and on methods of resolving the problems expeditiously.

SPACECRAFT ELECTRONICS and CONTROL

Communications and Tracking

The RAM C-II vehicle, designed and developed by the Langley Research Center, successfully measured the free electron and ion concentrations in the plasma surrounding a spacecraft during atmospheric reentry at a speed of approximately 25,000 feet per second. (Fig. 4-4) The results, which indicated that the theory was ineffective in predicting the distance from the spacecraft of the maximum ion concentration, will be used to refine the theory and establish a sound basis for predicting communication blackout in both manned earth reentry and entry into planetary atmospheres. The test was conducted at Wallops Island on August 22, using a Scout booster. (Fig. 4–5)

The extensive optical communication research and development program was granted space on the ATS-F spacecraft for a ground to spacecraft and return communication system experiment. The system to be developed by Goddard Space Flight Center will use carbon dioxide lasers and five inch antennas. ATS-G is expected to carry a similar system in that spacecraft so that tests can be conducted from satellite to satellite with ATS-F and thus prove out some of the concepts proposed for

the data relay satellite system. Laser communication offers two significant advantages: a high data rate and narrow beams that can be developed with very small antennas. On ATS-F, for example, the beam of the laser system using a 5-inch dish will be narrower and have a much higher gain than the S-band system using a 30-foot dish.

In telescope technology research, a 30-inch thin mirror was constructed and figured to test the concept of using thin deformable mirrors for large diffraction-limited space telescopes. This concept was developed in the search for a substitute for the thick mirrors with "waffle" reinforcing on the back used in earth-based telescopes which are too heavy to be launched. By

pushing and pulling the thin mirror, it can be made to retain the proper shape. Many small force elements attached to the back of the mirror do the pushing and pulling. They are connected to a laser measuring device in front of the mirror which continually senses its inaccuracies and causes the force elements to apply the proper force in the proper place to correct the observed errors. Such a large space telescope would be able to see much deeper into space than existing ground-based telescopes and might even be able to see the edge of the universe and thus add to knowledge of the forming of the cosmos. (Fig. 4-6)

The first space test of convolutional encoding and sequential decoding went into operation on the Pioneer IX spacecraft which was launched in November. The new deep space coding technique makes it possible to receive telemetering signals from a spacecraft at about a 40 per cent increase in range over uncoded systems or to send twice the amount of data in a given time at the same range. The system requires very little additional equipment on the spacecraft and uses existing computers on the ground to decode the messages and thus separate them from the high ambient noise. (Fig. 4-7)